In the production of nonwoven spun-bond webs, the apparatus can have a spinneret through which the thermoplastic synthetic resin filaments are forced, thereby forming a spun filament curtain which descends through a spinning and drawing shaft. The filament curtain is collected on a continuously movable receiving belt which is perforated or otherwise foraminous so that air can be drawn through this band by a suction device therebelow.
There are several systems which have been developed for the production of spun bond utilizing such an apparatus and the present invention is concerned with a system which can be described as a quiescent pressure or stationary pressure principle. The stationary pressure principle describes a process air system in which the process air is fed to the upstream end of the shaft at an inlet section in which a predetermined static pressure is maintained, i.e. the pressure air feed is proportioned to the air which passes downwardly through the shaft with the curtain so that the air in this portion of the shaft is practically static or at rest and the aforementioned static pressure and quiescent conditions are maintained.
The process air, of course, does pass downwardly through the shaft, accelerating in a convergent intermediate section below the inlet section before passing through the stretching section which can be of constant cross section and before finally emerging before a diffuser which flares outwardly and downwardly. The section below the belt also contributes to the draw upon filament and the movement of the air through the shaft. The stationary pressure principle with which the present invention is concerned can be contrasted with the driving jet principle in which nozzles are provided to generate high velocity jets which entrain the filaments downwardly.
The process air, of course, can also be referred to as cooling air and, in prior art systems utilizing the stationary pressure system, the process air is admitted transverse to the spun filament curtain in the inlet section of the spinning and drawing shaft.
The spinneret can be a perforated plate having an array of bores forming respective spinning nozzles and from which the spun filaments emerge. When reference is made herein to the contours of the shaft, it will be understood that these contours are as seen in a vertical section through the shaft in a plane perpendicular to the longitudinal dimension thereof and hence transverse to the horizontal longitudinal dimension of the curtain. The spinneret is customarily of rectangular configuration so that the array of orifices is elongated horizontally and hence the curtain itself, in a horizontal plane is elongated in a particular direction. The vertical section in which the contours of the shaft are defined is a vertical section perpendicular to this horizontal longitudinal dimension.
The apparatus of the foregoing type has been found to be highly effective in the production of spun bond but from the point of view of energy utilization can be improved. Indeed, we have found that it is possible to significantly improve the transfer coefficient, i.e. the quotient formed between the process air velocity and the spun filament velocity which corresponds to a constant drawing value averaged over all of the filaments of the spun filament curtain. This quotient generally is between 2.4 and 4 in conventional apparatus, i.e. the air speed is 2.4 to 4 times higher than the maximum spun filament velocity and thus its drawing value. The efficiency of the system is thus amenable to significant improvement.